1. Field of the Invention
The present invention relates to improvements in direction finding systems, and more particularly pertains to a Mobile Directional Comparator for use with a communications receiver and two port antenna system to process signal patterns to obtain directional information. The two received radio frequency signals from an unknown rf signal source are modulated with two low frequency sine waves combined together in a hybrid circuit, carried through the receiver as a normal rf signal, and the intermediate frequency output of the receiver is processed to extract the phase and amplitude information to drive a directional indicator.
2. Description of the Prior Art
In the field of direction finding receiving systems, it has been a general practice to employ a square wave modulation system to carry directional information through the receiver whereupon the IF output of the receiver is processed to extract directional information for the purpose of driving a directional indicator. Such devices have been unsatisfactory in that there is disturbance to the information on the received signal such as hash, hum, etc. Depending upon the modulation percentage of the square wave, the information on the received signal is unintelligible. As a result, it is difficult to simultaneously monitor the signal, record the signal, and direction find the signal radiating from the unknown rf signal source. Square wave modulation due to its hum, hash, etc. makes the information on the signal unintelligible, and although it is possible to direction find the received signal from the unknown rf signal source, it is not possible to simultaneously recover the information contained on the signal carrier. Further, some modulation systems require special antennae that can only be used over a very limited frequency range. The antenna might not lend itself readily from one direction finding application to another as the frequency of the unknown rf signal source might be different. Also, some modulation systems are unique to one particular communications receiver. It is therefore impossible to change communications receivers readily to cover different portions of the frequency spectrum. If it is necessary to cover a different portion of the frequency spectrum, it is necessary to not only use a different communications receiver, but also use a different modulation system which make rapid frequency changes impossible.
In a U.S. Pat. issued to Cotuno, 2,997,710, a direction finding system utilizes a square wave lobe switch to switch the termination and field patterns of each end of a rhombic antenna thereby modulating the incoming signal with a square wave of predetermined frequency. The signal that is modulated is fed to a receiver which filters the radio frequency carrier from a composite signal, and the remaining square wave signal is fed to an amplifier which functions to rotate the antenna in a direction such that the unknown rf signal received in each of the two cardioid field patterns has the same amplitude. Through a feedback loop, the rhombic antenna is rotated until the two cardioid field patterns have the same strength resulting in the direction of signal reception.